The traditional blackboard eraser is composed of cotton cloth. The most serious problem lies in the fact that the chalk dust flies in the air during the erasing. Moreover, the excess chalk dust adheres to the chalk eraser and not only is it difficult to clean it, but more chalk dust will be flying in the air during the cleaning. Therefore, the air will be polluted with harm to human health. The electric blackboard eraser has been proposed which has solved the defect of cleaning chalk dust, but it is not economical because of heavy equipment and energy requirement and is not popular. On Mar. 28, 1982 Mr. Chong Sun Yi received the U.S. Pat. No. 4,080,683 for Chalk Board Eraser and the blackboard eraser has obviously been improved. In this invention, the rollers are used for erasing the blackboard chalk dust. Moreover, there are brushes on one side of the rollers. When the rollers are erasing the blackboard, the brushes brush the chalk dust into a closed container. Furthermore, the gear system is installed on one side of the rollers for the purpose of providing means for slowing the rotational speed of the roller, so that there is sliding between the roller and the chalkboard, when the eraser is moved across the chalkboard in a direction transverse to the roller axle. The drawback of the above-mentioned device lies in the fact that there is only one piece roller. Furthermore, the case is triangular, and the roller is located at one angle of the triangle. Therefore, when it is used, the force is not distributed equally and the eraser is not stationary. In addition, Mr. Pang Hiang Seng has received U.S. patent Ser. No. 8,039,382 for Blackboard Duster, which uses two rollers, and a piece of a dusting member is installed between two rollers and passes over two rollers. It uses a dusting member as the erasing tool so that the contact area becomes larger. When the blackboard is erased, the dusting member can erase only the blackboard due to no special design for the rollers, so that it has no sliding effect as shown in U.S. Pat. No. 4,080,683, and it has no good erasing effect. One feature of the present invention lies in the fact that a movable receptacle is installed on one side of the box, and a swing door connected by a hinge element is installed between the receptacle and box. It can be opened only in one direction.